Phytoplasmas are a unique group of bacteria that exist as obligate parasites within plants and the insects that transmit them. These microorganisms are distinct from typical bacteria due to their lack of a rigid cell wall. They are notably small and cannot be cultured in artificial laboratory settings. Phytoplasmas reside primarily in the phloem tissue of infected plants, which is the system responsible for transporting sugars and nutrients throughout the plant.
Phytoplasma Transmission and Infection
Phytoplasmas spread from diseased plants to healthy ones primarily through the feeding activities of sap-sucking insects, particularly leafhoppers and psyllids. These insects act as vectors, acquiring the phytoplasma when they feed on the phloem of an infected plant. Once ingested, phytoplasmas multiply within the insect, migrating to its salivary glands. The insect then transmits the phytoplasma to a new plant by injecting it into the phloem during feeding.
The transmission process is persistent, meaning that once an insect acquires the phytoplasma, it can continue to transmit it for the remainder of its life. While insect vectors are the most common means of spread, phytoplasmas can also be transmitted through less frequent methods, such as grafting infected plant material onto a healthy plant.
Identifying Phytoplasma Diseases in Plants
Phytoplasma infections manifest through diverse symptoms, often leading to significant visual changes and reduced plant health. One striking symptom is “witches’ broom,” characterized by the abnormal proliferation of dense, broom-like clusters of shoots from a single point. This symptom is often accompanied by reduced leaf size and plant stunting. Another symptom is “phyllody,” where flower parts transform into leaf-like structures, preventing normal flower and fruit development.
“Virescence” is also common, involving the abnormal greening of flower petals. Beyond these transformations, infected plants often exhibit general yellowing of leaves (chlorosis) and stunted growth. For example, “Aster yellows,” caused by Candidatus Phytoplasma asteris, affects over 300 plant species, including carrots and coneflowers. Infected carrots may develop hairy roots and a bitter taste, while coneflowers often show deformed, green tufts of leaves.
Another notable case is “Lethal yellowing” in coconut palms, caused by Candidatus Phytoplasma palmae. Initial symptoms include the premature dropping of coconuts, followed by the blackening and death of flower stalks. The fronds then yellow, starting from the older leaves and progressing upwards, eventually leading to the collapse of the spear leaf and the death of the entire palm, often within three to six months.
Management and Prevention Strategies
There is currently no direct cure for a plant once it becomes infected with phytoplasma. Therefore, management focuses heavily on preventing the spread of the disease and minimizing its impact. A primary strategy involves the prompt removal and destruction of any plants showing symptoms of phytoplasma infection to eliminate sources of the pathogen and prevent further transmission.
Controlling insect vector populations is another important preventive measure. This can involve using insecticides to reduce sap-sucking insects like leafhoppers and psyllids. Physical barriers, such as netting or insect-exclusion screening, can also protect high-value crops and nursery stock from vector access. Planting certified disease-free nursery stock ensures new plants are not already carrying the phytoplasma.
Good field sanitation practices extend to managing weed species, as many weeds can act as alternative hosts for both phytoplasmas and their insect vectors. Removing these weeds reduces potential pathogen and vector reservoirs. Selecting plant varieties known to possess resistance or tolerance to specific phytoplasma diseases offers a long-term, environmentally sound approach to disease management. Research continues to identify and develop resistant genotypes for various economically important crops.